This invention relates to a developing device for use in image forming apparatus, such as electrostatic copying machines and facsimile apparatus, particularly a developing device including a developing unit disposed around a photoreceptor which is able to insert and draw along the rotation axis of the photoreceptor and a developing sleeve of which is able to move to from the photoreceptor.
In such type of developing device, there has been a developing device in the form of a cartridge which is removably set in the copying machine so as to enable developing units holding different color developers to replace with one another and make maintenance of developing units accessible, for example, a developing device of Japanese Unexamined Patent Publication No. 22907/60. The conventional device disclosed in the above publication is so constructed as to insert the unit into and draw out of apparatus in a vertical direction. The unit is inserted and drawn through an insertion hole provided at the same height as an original plate of the apparatus. The height is suitable for copying operation but considerably high for inserting and drawing the unit. Also, since the developing unit is a fairly heavy, handling of the developing unit is not easy. In the case of a copying machine equipped with a movable optical system for illuminating an original in an upper portion thereof, an additional space is necessary in order to provide a passage for inserting and drawing the unit in the vertical direction.
When providing a plurality of developing units holding different color developers respectively around a photoreceptor, they are generally arranged in a vertical direction. In the case of the above-mentioned developing device in which the unit is vertically inserted and drawn, as far as an upper-arranged developing unit is drawn out of the apparatus, a lower-arranged developing device can not be drawn. Accordingly, the conventional device needs a troublesome operation.
There has been another developing device which is constructed in such a manner that when setting the a developing unit in the apparatus, the unit is held near the photoreceptor so as to make developing executable, when inserting and drawing the unit, the unit is held apart from the photoreceptor so as to prevent the developing unit from striking the photoreceptor and then marring it, for example, a device of Japanese Unexamined Utility Model Publication No. 52746/56. In the device of the Utility Model Publication, the whole developing unit is moved in a horizontal direction to obtain a given distance between the photoreceptor and the unit. Accordingly, since apparatus equipped with the device needs an increased space, it is difficult to produce a compact apparatus.
Furthermore, heretofore has been known a developing device including a plurality of replaceable developing units provided around a photoreceptor to make multicolor copying effectable, the unit having a rotatable developing sleeve of non-magnetic material, and a magnet so rotatably provided in the sleeve as to render the rotation angle of the sleeve change according to whether or not the unit is put into operation. More specifically, in a unit put into developing operation, the magnet in the developing sleeve is rotated so that the main pole of the magnet faces the photoreceptor and then a magnetic brush of developer is formed on the portion of the sleeve surface which faces the photoreceptor. On the other hand, in a unit suspended from developing, the magnet in the developing sleeve is rotated so that the main pole of the magnet does not face the photoreceptor and then a magnetic brush of developer is kept away from the photoreceptor to prevent the developer of the suspended unit from straying into the developer being put into developing. Consequently, the device enables monocolor developing and superposition developing and further multicolor momocolor developing and multicolor superposition developing by replaceing developing units holding developers of different colors with one another.
However, this conventional device is very complicated in construction because the magnets in the sleeves is necessary to be rotated oppositely by the use of a solenoid or the like. It will be seen that if the device is constructed in such a manner that a trough of a magnetic brush of developer faces the photoreceptor with a little rotation of the magnet, the construction of the magnet rotating means becomes simpler. In this case, however, there is a likelihood that since the distance betweeen the seleeve and the photoreceptor is small, toner of the developer held in the unit suspended form developing is electrostatically attracted to an latent image formed on the photoreceptor and consequently toner of a part of developer different from the developer being used is developed. Also, there is a likelihood that since the magnet is rotated by electrical means, if the electrical means fails to operate properly, the magnetic brushes formed on all the developing sleeves face the photoreceptor, or the troughs of the brushes formed on all the sleeves face the photoreceptor, consequently the toners of different developers coming into contact with the photoreceptor at the same time, or no toners coming into contact with the photoreceptor.
Furthermore, the device has been greately demanded which is simple in construction and can be changed from a first state that a developing sleeve is put near a photoreceptor to make developing executable to a second state that the sleeve is moved away from the photoreceptor to make developing impossible and vice versa when the unit is set in apparatus in order to prevent the following incidents: incident that in operation stages outside the developing stage, such as warming-up stage and cleaning stage, the photoreceptor rotates with the photoreceptorin contact with a magnetic brush and the surface of photoreceptor consequently is scratched; incident that the developer comes into an aggregate with the developers being in contact with the photoreceptor due to a fact that the apparatus is suspended for a long period of time and then impairs the photoreceptor surface.
It is an object of the present invention to overcome the aforementioned problems.
Other objects and advantages of the present invention will become apparent from the following description of the present invention.